Having commissioned Willys, Ford and Bantam to build the 4500 jeeps (1500 each) in March 1941, the Motor Transport Board set up a project under the direction of the National Defense Research Committee (NDRC) to be designated ‘QMC-4 1/4 ton amphibian’. The Marmon-Herrington Co. (military vehicle specialists) in conjunction with boat builders Sparkman & Stephens and the Ford Motor Company undertook this work for the NDRC involving designing a conversion based on the 1/4 ton road vehicle. The aim was to have the vehicle in service in time for the first landing operations planed for September / October 1942.

Design and development work on a suitable prototype hull by Sparkman & Stephens was completed by the end of August by which time the standard design for jeeps had been introduced. However, conversion proved to be a difficult and time consuming task and the first amphibian QMC-4 vehicles were not available for river trials until February 1942.

Jeeps were now in full scale production by both Willys and Ford. Eager to have an amphibious version for the landings later that year, Generals responsible for the decision to proceed with an order were guided more by the concept rather than the need for the same extensive trials that the jeep had been through. By April the QMC-4 had been tried off a beach for the first time and an order for the first 5000 vehicles was placed with Ford based on their large scale production facilities. The Ford GPA Amphibian or Seep (Sea Jeep) based on the GPW chassis was born.

The first production run was delivered very quickly and there was little opportunity for experience in the field to be fed back to improve the design (as had been the case with the jeep). There were a number of design faults which, although not serious, meant that it required a lot of maintenance and lacked real effectiveness. It was not until November 1942 that the first of a whole string of necessary modifications filtered through to the production line. Modifications were still being made when production was finally ended in June 1943 by which time only 12,778 had been built.